The RPer guide about choosing Intra Ocular Lens for cataract surgery
Intra ocular lens (known as IOLs) are artificial replacements of the human cristaline. A cataract surgeon will replace the clouded lens in a cataract patient with an IOL. There are a few different types of IOLs: 1) monofocal lenses (targeted for a specific distance). With an operated eye, if the surgeon is able to precisesly calculate the power of the IOL needed for the eye, the patient will be able to see clearly (without glasses) to a specific distance. The surgeon can "target" the operation to offer best vision for distance (0 diopters or "plano") or for close vision (the eye will be myopic after surgery between -1.5 and -3 diopters). For any other distance (except the targeted distance) the patient will need different glasses (or better yet, progressive glasses). The patient and surgeon will discuss the following choices: 1a) Both eyes will have 0 diopters (plano) after surgery. If the surgery will hit the target (best case scenario), this will ensure good distance vision without glasses. For close and intermediate vision the patient will use two pairs of glasses (or bifocals) or progressive glasses. 1b) Both eyes will me myopic after surgery (between -1.5 and -3 diopters, usually -2). If the surgery will hit the target (best case scenario) the patient will have some close vision without eyeglasses (let's say between 30 cm and 70 cm or 12 inches to 27 inches). For intermediate and distance vision the patient will have to use two pairs of eyeglasses or bifocals or progressive eyeglasses. 1c) One eye will target plano (or something close to 0 - like -0.75 diopters for distance vision) and the other eye will be myopic to offer some close vision. This is called "monovision" (or little monovision if the difference between eyes is not so big). At any distance the brain will get an image in focus and a blurred image and should "choose" the image that is in focus. If the brain adapts well to this situation the patient has some degree of "multi distance" vision without any glasses. But this could be a gamble because the patient loses some depth perception and could have problems adjusting to monovision (some patients are not able to adjust). Monovision may not be an option to some RP patients with affected visual fields (with some "holes" in the visual field) or macular degenerations. b) multifocal lenses (or defractive lenses) will "split" the incoming light in order to offer close and distance vision at the same time. For a patient without retina problems, It will not offer perfect distance and perfect close vision but could offer good vision without eyeglasses at both distances (eyeglasses may be required for more difficult tasks). Lots of patients may loose some contrast sensitivity and have night problems with multifocal IOLs like halos, stars, glare. RP allready has night vision so you should consider if multifocals are a viable option. c) acomodating or "presbyopia" lenses ( Crystalens). Those don't "split" the light but move a little inside the eye when you try to focus to near distance and give you distance and some intermediate distance. No light "splitting" means acomodating IOLs should offer better contrast and more light (and no low vision side effects) compared with multifocals. But the surgery is more difficult and may not be an option for some RP patients (not sure why, but I guess things like the prevalence of weak zonules for RP patients). I've searched for a few days for a protocol or any "scientific methodology" to choose between 1a) (target 0 diopters for good distance vision) and 1b) (target -2.0 diopters myopia for close vision). Is it just patient prefference or is there a "good practice" rule? I found anly the following: http://www.reviewofophthalmology.com/content/i/1650/c/30426/ "Suppose a retired librarian comes in with end-stage macular degeneration, bilateral macular scars and bilateral cataracts, with a BCVA of 20/100. Should you aim to leave the patient plano, myopic or hyperopic after cataract surgery? No human eye should ever intentionally be made hyperopic. .... Consider the alternative: If you’d made the patient plano in both eyes, targeting for distance, he’d need +5 D reading glasses. As any optician will tell you, a pair of +5 D reading glasses is expensive to manufacture. You’ll have to put prism in the glasses to bring the images into alignment, because looking through +5 D spectacles without prism will cause diplopia at near—the brain will require excessive fusion to merge the two images. By intentionally making the patient myopic at the time of cataract surgery, you’ve turned an inexpensive device into a powerful low-vision aid, improved the patient’s quality of life and reduced the economic burden for the patient and society—all because you took optics into consideration.How do you determine where you want the near point of accommodation to be for a given patient? Kestenbaum’s rule (for determining the necessary near add in a patient with compromised acuity) states that the inverse of the patient’s best Snellen visual acuity is the number of diopters of add required. In this case, the inverse of 20/100 is 100/20, or 5 D. The 2 D of myopia you’ve left the patient with, plus 3-D OTC reading glasses, can now provide the patient with the near vision he needs.If the librarian does a lot of reading and needs to see things up close, you should aim to leave him a little bit myopic—around -1.5 to -3 D, depending on his preferred reading distance and arm length. Why? Because by leaving the patient with about 2 D of myopia, the patient can now buy an inexpensive over-the-counter +2- to +3-D pair of reading glasses, and the combination will provide the full amount of magnified near vision the patient needs for near work. In essence, you’ve converted an inexpensive pair of over-the-counter reading glasses into a powerful low-vision aid for someone who would potentially be blind without it." So, if I get this correctly, if the RP patient shows some degree of macular degeneration (or if the central vision wasn't very good before cataract), choosing a myopic target around -2.0 D after cataract surgery could be better for close vision activities. (The close vision could be fine without eyeglasses or could be easily improved with inexpensive glasses). Maybe this is why my cataract surgeon has a "rule": "for high myopia patients, we always target -2.0D after surgery" (but gets no explanation why). I guess high myopia is associated with some degree of macula degeneration as well, so targeting myopic -2.0 could offer best chances in those situations. Other consideration about cataract surgery for RPers: - RP patients have a greater risk of PCO (and may require YAG after cataract surgery). It seems the rectangular edge design of some IOLs has a lower incidence of PCO, so if you can, choose a rectangular edge IOL. - if you shold choose between a hidrophilic and a hidrophobic IOL (all other arguments beeing equal), a hydrophobic IOL seems to be better for RP patients. - if your cataract is not problematic yet, you may want to know that some interesting technologies are on the way: research for "melting cataract with eyedrops" (not yet in clinical trials) or light adjustable lenses that could be adjusted after they are implated in your eyes to "fine tune" the after cataract refractive error (I think LAL is now in 3rd phase clinical trial).